Polarized-holographic filtering providing improved extinction ratio

ABSTRACT

A filtering technique for free space communication that features an improved extinction ratio by providing a filter that employs a bulk holographic transform function and a polarizing film. In this manner, a greater number of channels of communication may be provided in a unit volume while preventing unwanted cross-talk between the communication channels. To that end, the system includes a source of energy to direct energy along a path, a detector disposed in the path, and a filter. The filter has a surface upon which a polarizing film is disposed and a holographic transform function recorded throughout a volume thereon.

BACKGROUND OF THE INVENTION

The present invention relates to wireless, or free space, communication.Particularly, the present invention concerns channel discriminationtechniques suited for wireless free space interconnects.

Reliance upon wireless technology is increasing as the need to increasecomputational efficiency becomes salient. Specifically, improvedoperational characteristics of data links employ advancements inwireless communication systems to replace conventional hardwiredtechnology. A well-known example includes the replacement ofconventional hardwired telephony with wireless cellular technology. Thishas generated a need for improvement methodologies that move away fromtraditional RF wireless technology to optical technology.

U.S. Pat. No. 4,057,319 to Ash et al. discloses an optical interconnectsystem in which individual connections are made involving the passage oflight between a specific device in one array of optical devices and aspecific device in another array of optical devices. This is achievedvia a phase hologram plate of the transmission type fixed relative toeach array.

U.S. Pat. No. 5,140,657 to Thylen discloses a device for opticallycoupling an optical fiber, forming part of an optical communicationsystem, to an optical semiconductor laser amplifier. Specifically, thesemiconductor laser amplifier has an input facet and an output facet,and the optical fiber has an end surface arranged opposite to at leastone of the facets. A diffraction optics element is disposed between theend surface of the fiber and the surface of the facet in order to adaptthe nearfield of the fiber end to the nearfield of the facet surfacewhile filtering the same to reduce spontaneous emission noise. Thediffraction optics element is described as being a phase hologram.

U.S. Pat. No. 6,072,579 to Funato discloses an optical pickup apparatusthat includes first and second light sources that selectively emit oneof first and second light beams, respectively. The first and secondlight beams are different in wavelength and are suitable for accessingfirst and second optical disks respectively. A coupling lens converts acorresponding one of the first and second light beams into a collimatedbeam. An objective lens forms a light spot on a corresponding one of thefirst and second optical disks by focusing the collimated beam. Aholographic optical element receives a reflection beam of the light spotfrom one of the first and second optical disks and provides holographiceffects on the reflection beam so as to diffract the reflection beam inpredetermined directions of diffraction depending on the wavelength ofthe reflection beam. A photo detector receives the reflection beam fromthe holographic optical element at light receiving areas and outputssignals indicative of respective intensities of the received reflectionbeam at the light receiving areas, so that a focusing error signal and atracking error signal are generated based on the signals. A drawbackwith the aforementioned optical interconnect systems is that eachcoupling device requires precise alignment of the optical elements toachieve efficient coupling of optical energy while avoiding cross-talkbetween adjacent channels.

What is needed, therefore, is an improved free space interconnecttechnique that reduces cross-talk between adjacent channels.

SUMMARY OF THE INVENTION

Provided is a communication system that features improved extinctionratio by providing a filter that employs a bulk holographic transformfunction and a polarizing film. In this manner, a greater number ofchannels of communication may be provided in a unit volume whilepreventing unwanted cross-talk between the communication channels. Tothat end, the system includes a source to direct energy along a path, adetector disposed in the path, and a filter. The filter has a surfaceupon which a polarizing film is disposed and a holographic transformfunction is recorded throughout a volume thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified plan view of a communication system in accordancewith one embodiment of the present invention;

FIG. 2 is a simplified plan view showing an apparatus for fabricatingthe filter apparatus shown above in FIG. 1, in accordance with oneembodiment of the present invention;

FIG. 3 is a perspective view of a subportion of a volume of the filterapparatus discussed above in FIGS. 1 and 2 showing a holographictransform function recorded therein;

FIG. 4 is a graphical representation showing charge distribution changesin the volume discussed above with respect to FIG. 3, in relation to theoptical energy impinging thereupon and the resulting strain in thematerial of the volume;

FIG. 5 is a perspective view demonstrating a first subportion of a bulkholographic transform function discussed showing a portion of thecharacteristics of optical energy impinging upon the volume discussedabove with respect to FIG. 3;

FIG. 6 is a perspective view demonstrating a second subportion of a bulkholographic transform function discussed showing a portion of thecharacteristics of optical energy impinging upon the volume discussedabove with respect to FIG. 3;

FIG. 7 is a perspective view demonstrating a third subportion of a bulkholographic transform function discussed showing a portion of thecharacteristics of optical energy impinging upon the volume discussedabove with respect to FIG. 3;

FIG. 8 is a perspective view demonstrating the resulting bulkholographic transform function recorded in a volume of material from thecombined characteristics shown above in FIGS. 5-7;

FIG. 9 is a simplified plan view showing encoding and decoding ofoptical energy in accordance with the present invention;

FIG. 10 is perspective view of the communication system shown above inFIG. 1, in accordance with an alternate embodiment;

FIG. 11 is perspective view of an array of the filters fabricated on aphoto-sheet shown above in FIG. 10;

FIG. 12 is a cross-sectional plan view of the optical communicationsystem shown above in FIG. 10, in accordance with the present invention;

FIG. 13 is a cross-sectional plan view of the optical communicationsystem shown above in FIG. 12, in accordance with an alternateembodiment of the present invention;

FIG. 14 is a simplified plan view showing sequential encoding anddecoding of optical energy in accordance with the present invention;

FIG. 15 is a perspective view of a compound holographic transformfunction in accordance with the present invention that may be employedas the filter discussed above with respect to FIG. 1;

FIG. 13 is a perspective view of the wavelength properties associatedwith the optical energy from which the interference pattern formed bythe apparatus shown in FIG. 9;

FIG. 14 is a perspective view of a holographic transform formed from therecording of the properties shown above with respect to FIGS. 11-13recorded in a subportion of a photo-sensitive sheet shown in FIG. 9;

FIG. 15 is a perspective view of a compound holographic transformed fromthe recording of two holographic transforms in a sub-portion of aphoto-sensitive sheet shown above in FIG. 9;

FIG. 16 is a cross-sectional view of a filter employed in thecommunication system shown above in FIG. 1, in accordance with analternate embodiment of the present invention;

FIG. 17 is a cross-sectional view of the filter employed in thecommunication system shown above in FIG. 1, in accordance with a secondalternate embodiment of the present invention;

FIG. 18 is a cross-sectional view of the filter employed in thecommunication system shown above in FIG. 1, in accordance with a thirdalternate embodiment of the present invention;

FIG. 19 is a cross-sectional view of a substrate on which the filterdiscussed above with respect to FIGS. 1 and 16-18 is fabricated;

FIG. 20 is a cross-sectional view of the substrate, shown above in FIG.19, undergoing processing showing a photo-resist layer disposed thereon;

FIG. 21 is a cross-sectional view of the substrate, shown above in FIG.20, undergoing processing showing a photo-resist layer being patterned;

FIG. 22 is a cross-sectional view of the substrate, shown above in FIG.21, undergoing processing after a first etch step; and

FIG. 23 is a cross-sectional view of the substrate, shown above in FIG.22, undergoing processing after a second etch step.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, shown is a communication system 10 including asource of optical energy 12, an optical detector 14 in datacommunication with the source of optical energy 12, with an filteringapparatus 16 disposed therebetween. The source 12 directs optical energy18 along a path 20 in which the optical detector 14 lies. The filteringapparatus 16 is disposed between the source 12 and the optical detector14 and filters optical energy propagating therethrough. In this manner,filtering apparatus 16 removes from unwanted characteristics from theoptical energy impinging upon optical detector 14.

The unwanted characteristics that may be removed from the optical energy18 includes amplitude wavelength and/or polarization information. Tothat end, filtering apparatus 16 has a bulk hologram recorded thereinthat defines a transform function, shown graphically as periodic lines26 for simplicity. The transform function 26 facilitates characterizingoptical energy 18 to have desired characteristics that may improvedetection of information contained in the optical energy 18, by theoptical detector 14. Specifically, optical energy 18 may function as acarrier wave and be modulated with information. Filtering is achieved bythe transform function selectively allowing specified characteristics ofthe carrier, e.g., optical energy 18, to pass therethrough and impingeupon optical detector 14.

Referring to both FIGS. 1 and 2, transform function 26 is recorded as aperiodic arrangement of the space-charge field of the material fromwhich filtering apparatus 16 is fabricated. The transform function 26 isrecorded employing a system 30 that includes a beam source 32 thatdirects a beam 34 a into wave manipulation optics 36, such as a ¼waveplate 38, so that a beam 34 b is circularly polarized. Beam 34 bimpinges upon polarizer 40 so that a beam 34 c propagating therethroughis linearly polarized. Beam 34 c impinges upon a Faraday rotator 42 thatchanges birefringence properties to selectively filter unwantedpolarizations from beam 34 c. In this manner, a beam 34 d egressing fromthe rotator 42 is linearly polarized. Beam 34 d impinges upon a beamsplitter 44 that directs a first subportion 34 e of beam 34 d onto aplanar mirror 46. A second subportion 34 f of beam 34 d passes throughsplitter 44. The first and second subportions 34 e and 34 f intersect atregion 50 forming an optical interference pattern that is unique in bothtime and space. The material from which filtering apparatus 16 isformed, photosensitive sheet 52, is disposed in the region so as to beexposed to the optical interference pattern. The interference patternpermeates the photosensitive sheet 52 and modulates the refractive indexand charge distribution throughout the volume thereof. To that end,sheet 52 may be formed from any suitable photo-responsive material, suchas silver halide or other photopolymers. Other materials from whichsheet 52 may be formed include LiNbO₃, LiTaO₃, BaTiO₃, KnbO₃, Bil₂SiO₂₀,Bi₁₂GeO₂₀, PbZrO₃, PbTiO₃, LaZrO₃, or LaTiO₃.

Referring to FIGS. 2, 3 and 4, the modulation that is induced throughoutthe volume of the photosensitive sheet 52 is in accordance with themodulation properties of the first and second subportions 34 e and 34 f.A subportion of the aforementioned volume is shown as 60. Across-section of volume 60 is shown as 64. An interference pattern,shown for simplicity as 66, is produced by beams 34 f and 34 e.Interference pattern 66 induces changes in refractive indices of volume64 based on the spatial modulation of photo-currents that results fromnon-uniform illumination. Charges such as electrons 68, or holes,migrate within volume 64 due to diffusion and/or drift in an electricfield present therein, referred to as photo-excited charges. Thegeneration of photocurrents at low beam intensity depends on thepresence of suitable donors. The photo-excited charges, which areexcited from the impurity centers by interference pattern 66, arere-trapped at other locations within volume 64. This produces positiveand negative charges of ionized trap centers that are re-excited andre-trapped until finally drifting out of the region of volume 64 uponwhich the interference pattern 66 impinges. This produces a chargedistribution within volume 64, shown by curve 70. Charge distribution 70creates a strain through volume 64, shown by curve 72 that producesregions of negative charge concentration 74 and regions of positivecharge concentration 76. The resulting space-charge field between theionized donor centers and the trapped photo-excited charges modulatesthe refractive indices, which is shown graphically by curve 78.

Referring to FIGS. 2 and 4-8, shown are exemplary data associated withthe interference pattern generated by the superposition of the first andsecond sub-portions 34 e and 34 f. Datum 80 shows one of the dimensionsrecorded in sheet 52. Specifically, datum 80 is a three-dimensionalrepresentation of the amplitude components of the interference pattern.Datum 82 is phase components associated with the interference pattern.Datum 84 is the wavelength components associated with the interferencepattern.

Recorded in a sub-portion of sheet 52, data 80, 82 and 84 define ahologram 88 that is defined throughout the entire bulk or volumetricthickness, v_(δ), measured between opposing sides of volume 64. Thevolumetric thickness, v_(δ), is defined to be the thickness required torecord a complete holographic transform function. It has been determinedthat, for a given material, the volumetric thickness, v_(δ), isinversely proportion to the wavelengths of first and second sub-portions34 e and 34 f that create the interference pattern. A volumetricthickness, v_(δ), as little as several microns was found suitable forrecordation of a single holographic transform in the near-infraredoptical frequencies. With the appropriate volumetric thickness, v_(δ),all of the physical properties associated with the photonic orelectromagnetic waves of the interference pattern, e.g., spatial andtemporal (phase) aspects, wavelength, amplitude, polarization, etc. arestored in volume 64. Holographic transform 88 functions as a gateway toprovide real-time and near real-time optical filtering and encoding.

Referring to FIG. 9, a wavefront 18 a is emitted by optical energysource 12 having a signal modulated thereon, shown as curves 18 b and 18c. After propagating through filtering apparatus 16, transform function26 operates on wavefront 18 a to rearrange the electromagnetic fieldsassociated therewith, thereby encoding the same. Encoded wavefront 16 aincludes the modulation 18 b and 18 c. However, in order to perceive theinformation associated with the modulation, the encoded wavefront 16 ashould be decoded. This requires propagating encoded signal 16 a througha transform function that is substantially identical to transformfunction 26. To that end, an additional filtering apparatus 116 havingthe same transform function 26 recorded therein should be placed betweena detector 14 and encoded wavefront 16 a. Upon propagating throughfiltering apparatus 116 encoded wavefront 16 a is unencoded, therebyrendering unencoded wavefront 18 a and all the information contained inmodulation 18 b and 18 c. Thus, it is seen that the inverse transform ofthe transform function 26 is the transform function 26 itself. Thus,propagating a wavefront through an even multiple of a single transformfunction, the original wavefront may be maintained. Conversely,propagating wavefront through an uneven multiple of a single transformfunction results in an encoded wavefront, which is virtually impossibleto detect, much less demodulate, without unencoding the same. In thismanner, superior beam-sensor discrimination may be achieved.

Referring to FIG. 10, beam-sensor discrimination provided by the presentinvention is beneficial to a multi-channel optical communication system310. One example of optical communication system 310 includes an array312 of optical transmitters, shown generally as 312 a-312 p, and anarray 314 of optical detectors, shown generally as 314 a-314 p. Theoptical transmitters 312 a-312 p generate optical energy to propagatealong a plurality of axes, and the optical receivers 314 a-314 p arepositioned to sense optical energy propagating along one of theplurality of optical axes. Specifically, the array 312 is an (X×Y) arrayof semiconductor lasers that produce a beam that may be modulated tocontain information. The array 314 may comprise of virtually any opticaldetector known, such a charged coupled devices (CCD) or charge injectiondetectors (CID). In the present example, the array 314 comprises of CIDsarranged in an (M×N) array of discrete elements. The optical beam fromthe each of the individual emitters 312 a-312 p may expand to impingeupon each of the detectors 314 a-314 p of the array 314 if desired.Alternatively, the optical beam from each of the individual emitters 312a-312 p may be focused to impinge upon any subportion of the detectors314 a-314 p of the array 314, discussed more fully below. In thisfashion, a beam sensed by one of the detectors 314 a-314 p of the array314 may differ from the beam sensed upon the remaining detectors 314a-314 p of the array 314. To control the wavefront of the optical energyproduced by the transmitters 312 a-312 p, the filtering apparatus 16,discussed above with respect to FIGS. 1-8 may be employed as an array ofthe filtering apparatuses 416, shown more clearly in FIG. 11 as array400.

Specifically, referring to FIGS. 11 and 13, the individual filteringapparatuses 416 of the array are arranged to be at the same pitch andsizing of the array 312. The numerical aperture of each of the filteringapparatuses 416 of the array 400 is of sufficient size to collectsubstantially all of the optical energy produced by the transmitters 312a-312 p corresponding thereto. In one example, the array 400 is attachedto the array 312 with each lens resting adjacent to one of thetransmitters 312 a-312 p. To provide the necessary functions, each offiltering apparatuses 416 of the array 400 may be fabricated to includethe features mentioned above in FIGS. 1-8. As a result, each of thefiltering apparatuses 416 b of the array may be formed to havingfunctional characteristics that differ from the remaining filteringapparatuses 416 of the array. In this manner, each beam produced by thearray 312 may be provided with unique properties, such as wavelength,amplitude and polarization. This facilitates reducing crosstalk andimproving signal-to-noise ratio in the optical communication system 310.

Specifically, the filtering apparatus 316 may include an additionalarray 400 b of filtering apparatuses 416 b that match the pitch of theindividual detectors 314 a-314 p of the array 314, shown more clearly inFIG. 13. The filtering apparatuses 416 b may be fabricated to providethe same features as discussed above with respect to array 400, shown inFIG. 10.

Referring to FIGS. 10, 11 and 13 each of the transmitters 312 a-312 p ofthe array 312 would then be uniquely associated to communicate with onlyone of the detectors 314 a-314 p of the array 314. In this manner, thetransmitter 312 a-312 p of the array 312 that is in data communicationwith one of the one of the detectors 314 a-314 p of the array 314 woulddiffer from the transmitters 312 a-312 p in data communication withremaining detectors 314 a-314 p of the array 314, forming atransmitter/detector pair that is in optical communication.Communication between the transmitter detector pair is achieved byhaving the properties of the filtering apparatuses 416 in array 400associated with the transmitter match the properties of the filteringapparatuses 416 b in array 400 b associated with the detector. Forexample were the filtering apparatuses 416 associated with transmitter312 a to match the properties of filtering apparatuses 416 b associatedwith detector 314 c, the optical energy produced by transmitter 312 acould be sensed by detector 314 c. Assume no other detector hasfiltering apparatuses 416 b associated therewith that have propertiesmatching the properties of the filtering apparatuses 416 associated withtransmitter 312 a. Then detector 314 c would be the only detector ofarray 314 capable of sensing optical energy from transmitter 312 a. Thisresults from the inherent properties of holographic transforms,discussed more fully below. It should be seen that in addition tofiltering, the holographic transform provides security againstunauthorized sensing of optical energy. In this manner, informationmodulated on the optical energy produced by transmitter 312 a may onlybe perceived by detector 314 c. This is also discussed more fully below.

It should be understood, however that one of the transmitters 312 a-312p might be in data communication with any number of the detectors 314a-314 p by multiple filtering apparatuses 416 b matching the propertiesof one of the filtering apparatuses 416. Similarly, one of the multipletransmitters 312 a-312 p may be in optical communication with one ormore of the detectors 314 a-314 p by appropriately matching thefiltering apparatuses 416 to the filtering apparatuses 416 b.

In one example, superior performance was found by having the array 314sectioned into (m×n) bins, with each bin corresponding to a particularpolarization and/or wavelength that matched a particular polarizationand/or wavelength corresponding to a transmitter 312 a-312 p. Thus, werea beam from one or more of the transmitters 312 a-312 p to flood theentire (M×N) array 314 or multiple (m×n) bins, only the appropriatedetectors 314 a-314 p sense information with a very high signal-to-noiseratio and discrimination capability.

Additional beam-sensor discrimination may be achieved by employingtransmitters 312 a-312 p having different wavelengths or byincorporating up-conversion processes that include optical coatingsapplied to the individual transmitters 312 a-312 p or made integraltherewith. One such up-conversion process is described by F. E. Auzel in“Materials and Devices Using Double-Pumped Phosphors With EnergyTransfer”, Proc. of IEEE, vol. 61. no. 6, June 1973. In addition,coating one or more filtering apparatuses 416 of array 400 with apolarizing film provides further discrimination using polarizingdiscrimination. The combined effect of the transform function and thepolarizing improves the extinction ratio of either the transformfunction or the polarizing film by one order of magnitude or better. Forexample, a typical polarizing film providing an extinction ratio of 50to 100 may be increased to 1,000, or better, when employed inconjunction with the transform function in accordance with the presentinvention. Similar improvements in the extinction ratio of a transformfunction is realized with this combination. To that end, the polarizingorientation of the film should match the polarizing orientation providedby the transform function.

Referring to both FIGS. 2 and 13, filtering apparatuses 416 and 416 b,with differing transform functions are formed on differingphotosensitive sheets 52. Specifically, the transform function isdefined by the interference pattern formed by the first and secondsubportions 34 e and 34 f intersecting at region 50. This interferencepattern is unique in both time and space. As a result, each of thefiltering apparatuses formed on the sheet 52 would have substantiallyidentical holographic transform functions. To create filter apparatuseswith differing transform functions, an additional photosensitive sheet52 would be employed. Considering that the interference pattern isunique in both time and space, a subsequent sheet 52 disposed in region50 would have a differing transform function recorded therein thereonthan the transform function recorded on a sheet 52 at an earlier time.This is due, in part, to the time-varying fluctuations in theoperational characteristics of the various components of system 30. As aresult multiple sheets 52 are formed, each of which has a transformfunction associated therewith that differs from the transform functionassociated with the remaining sheets. After forming the aforementionedmultiple sheets, the filtering apparatuses on each of the sheets issegmented so that the same may be arranged proximate to one or moreemitters and one or more detectors, as desired.

Alternatively, or in addition, the Faraday rotator 42 may be rotated toprovide the lenses formed on the photosensitive sheet 52 with aholographic transform function that differs from the transform functionassociated with the lenses formed on a previous photosensitive sheet 52.

Referring to FIG. 13, it should be noted that the array 312 may comprisea single emitter 412 that produces sufficient beam width to impinge uponall of filtering apparatuses 416 of array 400. In this manner, the array400 of filtering apparatuses 416 is employed in the aggregate toincrease both the numerical aperture and enhance the signal to noiseratio, as well as to provide a multi-transform operation across thecross-section of the optical energy produced by single emitter 412.Employing the multi-transform operation over the cross-sectional area ofthe optical energy takes advantage of the properties of the holographictransforms recorded in each filtering apparatuses 416. Specifically, theholographic transforms in each of the filtering apparatuses function inthe aggregate to operate on the wavefront of the optical energy as anaggregate holographic transform to vary the wavefront, defining anencoded wavefront. The encoded wavefront may be returned to theun-encoded state, i.e., decoded, by having the same propagate through amatching aggregate holographic transform. To that end, the array ofdetectors 314 may comprise of a single detector to facilitate unencodingof the optical energy. This makes the present invention suitable for usewith free space interconnects over local area networks, wide areanetworks and metropolitan area networks, because multiple networks maycommunicate through a common volume of space without corrupting the dataassociated with the network.

Referring to FIG. 14, another property of the transform functionconcerns sequential encoding and decoding. As mentioned above, theinverse transform function of a holographic transform function is thefunction itself. As a result, multiple encoding may be facilitated toprovide increased beam-sensor discrimination. Specifically, assuming anoptical encoding system 415 comprising a first filtering apparatusincluding 416 a having a first transform function H₁ and a secondfiltering apparatus 417 a including a second transform function H₂.Propagating wavefront 418 a through first filtering apparatus including416 a would result in encoded wavefront 419 a. Propagating encodedwavefront 419 a through second filtering apparatus 417 a would furtherencode wavefront 419 a, yielding encoded wavefront 421 a. To decodewavefront 421 a to yield unencoded wavefront 418 a requires firstpassing wavefront 421 a through the second transform function to yieldwavefront 419 a. Thereafter, wavefront 419 a would propagate through thefirst transform function to yield unencoded wavefront 418 a. To thatend, a decoding system 415 b includes a third and fourth apparatus 417 band 416 b, respectively. Third filtering apparatus 417 b has a transformfunction associated therewith that is identical to transform functionH₂. Fourth filtering apparatus 416 b has a transform function associatedtherewith that is identical to transform function H₁. To decodewavefront 421 a, third filtering apparatus 417 b is positioned betweensecond filtering apparatus 417 a and fourth filtering apparatus 416 b.In this manner, wavefront 421 a first propagates through third filteringapparatus 417 b to be decoded by transform function H₂ forming wavefront419 a. Wavefront 419 a then propagates through filtering apparatus 416 bto be decoded by transform function H₁, thereby yielding wavefront 418a. Wavefront 418 a may then be sensed by a detector (not shown) toretrieve information contained therein.

Reversing the order of unencoding so that the first transform operatedon wavefront 421 a would yield unintelligible information, therebypreventing any information modulated on wavefront 418 a being unencoded.

Referring to FIG. 15, a property recognized with respect to theholographic transform functions is that two holographic transformfunction may be recorded in the an identical volume without interferingwith each other. As a result a compound holographic transform function500 may be recorded in which two or more independent holographictransform functions are recorded across a unit volume. Compoundholographic transform function 500 is shown having two holographictransform functions 500 a and 500 b recorded therein. It was determined,however, that the volumetric thickness, v_(δ), was also defined by thenumber of holographic transforms recorded in a unit volume formed in avolume. Specifically, it is found that were recording and retrieval ofmultiple and independent holographic transforms, e.g., numbering in thehundreds and thousands, desired, then several millimeters of volumetricthickness, v_(δ), would be required.

Referring to FIGS. 2 and 16, to relax the alignment tolerance betweenoptical energy source 12 and detector 14, filtering apparatus 16 may beprovided with a lensing function. In this manner, filtering apparatus 16may concurrently refract and filter optical energy 18. In this manner,filtering apparatus 16 defines a lens 22 having a bulk holographictransform function 26 recorded in substantially the entire volumethereof, through which optical energy will propagate. In this manner,the lens 22 and the bulk holographic transform function 26 areintegrally formed in a manner described more fully below. Although thesurface 28 of the lens 22 disposed opposite to the spherical arcuatesurface 24 is shown as being planar, the surface 28 may also be arcuateas shown in surface 128 of lens 122 in FIG. 17.

The refractory function of the filtering apparatus 16 facilitatesimpingement of the optical energy 18 onto the optical detector 14. Inthis manner, the precise alignment of the optical detector 14 withrespect to the source 12 and, therefore, the path 20 may be relaxed.

Referring to both FIGS. 2 and 18, were it desired to further control theshape of optical energy propagating through lens 22, a lens 222 may beformed with a Fresnel lens 228 disposed opposite to the sphericalsurface 224. In this manner, substantially all of the optical energypropagating through lens 222 may be focused to differing points,depending upon the wavelength of optical energy propagatingtherethrough. To that end, the Fresnel lens 228 includes a plurality ofconcentric grooves, shown as recesses 228 a 228 b and 228 c that areradially symmetrically disposed about a common axis 230. Thus, lens 222may have three optical functions integrally formed in a common element,when providing the bulk holographic transform function 226 therein.

To provide the aforementioned lensing function, the manufacturingprocess of photosensitive sheet 52 may include providing aphotosensitive layer 800 adhered to a sacrificial support 802, shown inFIG. 19. Examples of sacrificial layers include glass, plastic and thelike. The photosensitive layer 800 and sacrificial support 802 form aphotosensitive substrate 804. Typically, photosensitive layer 800 istens of microns thick. As shown in FIG. 20, a photo resist layer 806 isdeposited onto the photosensitive layer 800 and then is patterned toleave predetermined areas exposed, shown as 808 in FIG. 21, defining apatterned substrate 810. Located between exposed areas 808 are photoresist islands 812. Patterned substrate 810 is exposed to a lightsource, such as ultraviolet light. This ultraviolet light darkens thevolume of photo resist layer 800 that is coextensive with exposed areas808 being darkened, i.e., become opaque to optical energy. The volume ofphotosensitive layer 800 that are coextensive with photo resist islands812 are not darkened by the ultraviolet light, i.e., remainingtransparent to optical energy. Thereafter, photo resist islands 812 areremoved using standard etch techniques, leaving etched substrate 814,shown in FIG. 22.

Etched substrate 814 has two arcuate regions 816 that are located inareas of the photosensitive layer 800 disposed adjacent to islands 812,shown in FIG. 23. Arcuate regions 816 of FIG. 22 result from thedifference in exposure time to the etch process of the differing regionsof photosensitive layer 800.

Referring to FIGS. 2, 11 and 22, a subsequent etch process is performedto form array 400. During this etch process the support is removed aswell as nearly 50% of photosensitive layer 800 to form a very thinarray. Array 400 is then placed in the apparatus 30 and the bulkholographic transform functions are recorded in the arcuate regions 816that define the lenses, as discussed above. The Fresnel lens may also beformed on the lenses of the array 400 using conventional semiconductortechniques. Thereafter, the lenses may be segmented from the photoresistive sheet or M×N sub-arrays of lenses may be segmented therefrom.

Although the invention has been described in terms of specificembodiments, one skilled in the art will recognize that various changesto the invention may be performed, and are meant to be included herein.For example, in additional to the optical communication discussed above,the present invention may be employed for RF communication usingwavelengths in the range of one micron to one millimeters, inclusive.

In addition, instead of a transmissive filtering apparatus 16, areflective filtering apparatus may be employed. The present inventionwould be suited for use on storage media such as compact diskettes thatstore various information, e.g., audio content, video content,audio-visual content and the like. In this manner, a signal, eitheroptical or RF, would propagate into the filtering apparatus and bereflected back from the filtering apparatus through a common surface.

Further, instead of forming the arcuate regions 816 using standard etchtechniques, the same may be formed by exposing substrate 810, shown inFIG. 21, to thermal energy. In one example, substrate 810 isconvectionally heated, and photo resist layer 806 is patterned tocontrol the regions of photosensitive layer 800 that may expand.

In another example, the photosensitive layer is heated by conductionemploying laser ablation/shaping. Specifically, a laser beam impingesupon areas of photosensitive layer 800 where lens are to be formed. Thethermal energy from the laser beam causes the photosensitive layer 800to bubble, forming arcuate regions 816 thereon, as shown in FIG. 22. Inaddition, the holographic transform function has been found to beeffective in filtering electromagnetic energy outside of the opticalspectrum, e.g., in the microwave region. Therefore, the scope of theinvention should not be based upon the foregoing description. Rather,the scope of the invention should be determined based upon the claimsrecited herein, including the full scope of equivalents thereof.

1. A communication system comprising: a source of energy to propagate asignal along a communication path; a detector positioned in thecommunication path; and a filter having a surface with a polarizing filmdisposed thereon and first and second holographic optical elements eachof which has a holographic transform function disposed throughout thevolume of the filter, the filter being disposed in the path to filterthe signal in accordance with properties of the holographic transformfunction and the polarizing film to remove portions of the signal havingunwanted characteristics, defining a transformed signal to impinge uponthe detector, with the transform function associated with said firstholographic optical element matching the transform function associatedwith said second holographic element.
 2. The system as recited in claim1 wherein the characteristics are selected from a group consistingessentially of amplitude, polarization, wavelength and phase.
 3. Thesystem as recited in claim 1 wherein the filter is a transmissiveelement, allowing the signal to propagate between opposing surfacesthereof.
 4. The system as recited in claim 1 wherein the filter is areflective element, allowing the signal to enter and exit the elementthrough a common surface.
 5. The system as recited in claim 1 whereinthe signal is an optical signal.
 6. The system as recited in claim 1wherein the signal is an RF signal having a wavelength in the range ofin the range of 1 micron to 1 millimeter, inclusive.
 7. The system asrecited in claim 1 wherein the source of energy includes an array oftransmitters to generate a plurality of signals to propagate along aplurality of axes and the detector includes an array of receivers, eachof which is positioned to sense one of the plurality of signalspropagating along one of the plurality of axes and the filter includesan array of filters, each of which is disposed in one of the pluralityof axes, with a subset of the filters of the array having the polarizingfilm disposed on a surface thereof and the holographic transformdisposed in a volume of the filter.
 8. The system as recited in claim 1wherein the source of energy includes an array of transmitters togenerate energy to propagate along a plurality of axes and the detectorincludes an array of receivers, each of which is positioned to senseenergy propagating along one of the plurality of axes and the filterincludes a plurality of filters each of which has a holographictransform function within a volume thereof, with the plurality offilters being arranged in first and second arrays, the first array beingdisposed between the array of transmitters and the array of receiversand the second array being disposed between the first array and thereceivers.
 9. The system as recited in claim 8 wherein the holographictransform function associated with a subgroup of the filters of thefirst array, defining a transfer function, differs from the holographictransform function associated with the remaining filters of the firstarray of filters, and the holographic transform function associated witha subset of the filters of the second array matches the transferfunction.
 10. The system as recited in claim 1 wherein the filter is anoptical element has opposed sides with a spherical surface beingpositioned on one of the opposed sides and a planar surface beingdisposed on the remaining side of the opposed sides with the holographictransform function being disposed within a volume of the lens betweenthe spherical and the planar surfaces.
 11. The system as recited inclaim 1 wherein the filter is an optical element has opposed sides witha cylindrical surface being positioned on one of the opposed sides and aplanar surface being disposed on the remaining side of the opposedsides, with the holographic transform function being disposed within avolume of the lens between the cylindrical and the planar surfaces. 12.The system as recited in claim 1 wherein the filter is an opticalelement that has opposed sides with a spherical surface being positionedon one of the opposed sides and a rotary symmetric arrangement ofgrooves defining a Fresnel lens being disposed on the remaining side ofthe opposed sides with the holographic transform function being disposedwithin a volume of the lens between the spherical surface and theFresnel lens.
 13. The system as recited in claim 1 wherein the source ofenergy includes an array of optical transmitters to generate opticalenergy to propagate along a plurality of axes and the detector includesan array of optical receivers, each of which is positioned to senseoptical energy propagating along one of the plurality of optical axesand the filter includes an array of lenses, each of which is disposed inone of the plurality of axes and includes the arcuate surface with theholographic transform being disposed within a volume of the array oflenses.
 14. The system as recited in claim 1 wherein the source ofoptical energy includes an array of optical transmitters to generateoptical energy to propagate along a plurality of axes and the detectorincludes an array of optical receivers, each of which is positioned tosense optical energy propagating along one of the plurality of opticalaxes and the filter including a plurality of lenses having the arcuatesurface with holographic transform function being disposed within avolume thereof, with the plurality of lenses being arranged in first andsecond arrays, the first array being disposed between the array ofoptical transmitters and the array of optical receivers and the secondarray being disposed between the first array and the optical receivers.15. A communication system comprising: a source of optical energy topropagate a signal along an optical path; an optical detector positionedin the communication path; and an optical filter having a surface with apolarizing film disposed thereon and first and second holographicoptical elements each of which has a holographic transform functiondisposed throughout the volume of the optical filter, the optical filterbeing disposed in the optical path to filter the signal in accordancewith properties of the holographic transform function and the polarizingfilm to remove portions of the signal having unwanted characteristics,defining a transformed signal to impinge upon the detector, with thetransform function associated with said first holographic opticalelement matching the transform function associated with said secondholographic element.
 16. The system as recited in claim 15 wherein thesource of optical energy includes an array of optical transmitters togenerate optical energy to propagate along a plurality of axes and thedetector includes an array of optical receivers, each of which ispositioned to sense optical energy propagating along one of theplurality of optical axes and the optical system includes an array oflenses, each of which is disposed in one of the plurality of axes andincludes the arcuate surface with the holographic transform beingdisposed within a volume of the array of lenses.
 17. The system asrecited in claim 16 wherein the source of optical energy includes anarray of optical transmitters to generate optical energy to propagatealong a plurality of axes and the detector includes an array of opticalreceivers, each of which is positioned to sense optical energypropagating along one of the plurality of optical axes and the opticalsystem including a plurality of lenses having the arcuate surface withholographic transform function being disposed within a volume thereof,with the plurality of lenses being arranged in first and second arrays,the first array being disposed between the array of optical transmittersand the array of optical receivers and the second array being disposedbetween the first array and the optical receivers.
 18. A communicationsystem comprising: an array of optical transmitters to generate opticalenergy to propagate along a plurality of axes; an array of opticalreceivers, each of which is positioned to sense optical energypropagating along one of the plurality of optical axes; a first array ofrefractory lenses, each of which is disposed in one of the plurality ofaxes and has a first polarizing film disposed thereon and a firstholographic transform function recorded throughout a volume thereof tofilter from the optical energy unwanted characteristics, with the firstpolarizing film and first holographic transform function associated witha subgroup of the lens of the first array, defining a transfer function,differing from first polarizing film and the first holographic transformfunction associated with the remaining lenses of the first array oflenses; and a second array of refractory lenses, each of which isdisposed between the first array of lenses and the array of opticalreceivers to collect optical energy propagating along the one of theplurality of optical axes, with a subset of the lenses of the secondarray having a second polarizing film thereon and a second holographictransform function recorded in a second volume thereof, with the secondpolarizing film and the second holographic transform function matchingthe transfer function.
 19. The system as recited in claim 18 wherein thelenses of the first and second arrays have a spherical surface and anadditional surface disposed opposite to the spherical surface, with aFresnel lens being disposed on the additional surface.
 20. The system asrecited in claim 18 wherein the lenses of the first and second arrayshave a cylindrical surface and an additional surface disposed oppositeto the cylindrical surface, with a Fresnel lens being disposed on theadditional surface.